Many fields of applied biological sciences, notably the health care professions, utilize biocidal gasses to effect chemical sterilization. Chemical sterilization with a biocidal agent such as ethylene oxide is often utilized to sterilize heat sensitive goods that could not tolerate high temperature sterilization. For example, biocidal gas sterilization is routinely used for health care products such as bioabsorbable devices, catheters, introducers, stents and laparoscopes.
One type of biocidal gas sterilization is ethylene oxide sterilization, which is a well established method used by both hospitals and manufacturers of sterile goods. This process utilizes ethylene oxide gas adjusted to a certain humidity, temperature, and concentration in an inert gas in an enclosed chamber to kill microbial spores, vegetative bacteria and other microorganisms.
The penetration of the ethylene oxide, as well as the humidification and heating of the goods to be sterilized, is carried out more effectively if air is first evacuated from both the sterilization chamber and the packaged or wrapped goods. This evacuation of air and the addition of moisture is known as "conditioning" of the goods to be sterilized and is generally done prior to introduction of the ethylene oxide, which is subsequently introduced to the chamber to a predetermined pressure, usually above atmospheric pressure. These procedures are more fully described in U.S. Pat. Nos. 4,971,761; 4,203,942; and 4,671,936, all of which are incorporated by reference.
Another method of sterilization is gamma irradiation sterilization. See, for example, U.S. Pat. Nos. 5,800,542 and 5,554,437, all of which are incorporated by reference. The gamma irradiation of biologic materials under sterilization conditions is generally 25 kGy. However, most polymeric materials including biopolymers are sensitive to gamma irradiation. The polymer chains are degraded by chain scission due to the high energy gamma rays. Under most sterilization dose conditions, the net result of gamma irradiation is a reduction of molecular weight of the polymer. This gamma irradiation induced degradation can last for a long period of time by the entrapped free radicals produced within the polymeric material.
The main advantage of ethylene oxide sterilization over gamma irradiation sterilization, particularly in sterilizing sensitive materials such as bioabsorbable devices, is that ethylene oxide sterilization does not degrade the bioabsorbable polymer to any significant extent, whereas gamma irradiation sterilization does degrade it.
Ethylene oxide, while an efficient low temperature sterilant, is an irritant that must be purged to the maximum extent possible from the packs containing the articles to be sterilized, and from the articles themselves. Residue in the product which has been sterilized by ethylene oxide gas is a concern from a biocompatability and safety point of view, therefore, the amount of this residue has to be minimized.
The FDA requirements for small bioabsorbable polymer devices, i.e. less than 10 grams, is below 250 parts per million ("ppm") ethylene oxide. The normal ethylene oxide sterilization cycle leaves ethylene oxide residue levels of 1000-2400 ppm depending on the sterilization conditions. Removal of this residual ethylene oxide is routinely accomplished by an aeration procedure where the sterilized goods are aerated in a chamber for a period of time sufficient to remove substantially all traces of the sterilant gas.
It is well known in the art that both the initial ethylene oxide amount introduced into a sterilizer and its dwell time in the sterilizer affect the amount of residual ethylene oxide in the device, particularly for devices comprised of amorphous polymers.
What is needed is an improved method by which the amount of residual ethylene oxide can be reduced to acceptable levels, while not degrading sensitive materials, such as bioabsorbable devices.